Balloons for medical devices and fabrication thereof

ABSTRACT

Dilatation balloons and catheters including same are provided. The balloons are fabricated of a material such as a nylon or a polyamide material, and they have an inflated, non-distended working profile as well as a stretched inflated profile which is achieved by applying pressure through a dilatation catheter or the like that is in excess of that needed to achieve the inflated, non-distended profile and which is adequate to effect dilatation or the like up to a maximum pre-bursting pressure application. The maximum pre-bursting size of the balloon can be tailored depending upon the needs of the particular balloon within a wide range of possible maximum pre-bursting sizes.

This application is a continuation of application Ser. No. 08/475,530filed Jun. 7, 1995, now abandoned, which is a continuation ofapplication Ser. No. 178,765, filed Jan. 7, 1994 now U.S. Pat. No.5,449,371 which is a continuation of Ser. No. 07/958,033 filed Oct. 7,1992 now U.S. Pat. No. 5,304,197, which is a continuation of applicationSer. No. 07/561,747 filed Aug. 2, 1990 now U.S. Pat. No. 5,156,612,which is a continuation of application Ser. No. 07/452,713 filed Dec.19, 1989 now U.S. Pat. No. 5,108,415, which is a divisional ofapplication Ser. No. 07/384,723 filed Jul. 24, 1989 now U.S. Pat. No.4,906,244, which is a continuation-in-part of application Ser. No.07/253,069 filed Oct. 4, 1988 now abandoned.

BACKGROUND AND DESCRIPTION OF THE INVENTION

The present invention generally relates to balloons for medical devices,as well as to processes and equipment for forming same. Moreparticularly, the invention relates to medical or surgical balloons andto catheters incorporating them, such as those designed for angioplasty,valvuloplasty and urological uses and the like. The balloons exhibit theability to be tailored to have expansion properties which are desiredfor a particular end use. The expansion property of particular interestis the amount or percentage of expansion or stretching beyond anon-distended inflated size at which the balloons are inflated to removefolding wrinkles but are not stretched. Balloons which are especiallysuitable in this regard are made of nylon or polyamide tubing that hasbeen formed into the desired balloon configuration.

Catheter balloons and medical devices incorporating them are well-knownfor use in surgical contexts such as angioplasty procedures and othermedical procedures during which narrowings or obstructions in bloodvessels and other body passageways are altered in order to increaseblood flow through the obstructed area of the blood vessel. For example,in a typical balloon angioplasty procedure, a partially occluded bloodvessel lumen is enlarged through the use of a balloon catheter that ispassed percutaneously by way of the arterial system to the site of thevascular obstruction. The balloon is then inflated to dilate the vessellumen at the site of the obstruction.

Essentially, a balloon catheter is a thin, flexible length of tubinghaving a small inflatable balloon at a desired location along itslength, such as at or near its tip. Balloon catheters are designed to beinserted into a body passageway such as the lumen of a blood vessel, aheart passageway, a urological passageway and the like, typically withfluoroscopic guidance.

In the past, medical device balloon materials have included balloonshaving a wall thickness at which the material exhibits strength andflexibility that allow inflation to a working diameter or designatedinitial dilation diameter which, once achieved, is not surpassable toany significant degree without balloon breakage or substantiallyincreasing the risk of balloon breakage. Balloons of these materials canbe characterized as being substantially non-distensible balloons thatare not stretchable, expandable or compliant to a substantial extentbeyond this working diameter. Such substantially non-distensibleballoons can be characterized as being somewhat in the nature of paperbags which, once inflated to generally remove folding wrinkles, do notfurther inflate to any significant degree. Polymeric materials of thissubstantially non-distensible type that are used or proposed for use asmedical balloons include polyethylene terephthalates.

Other types of materials, such as polyvinyl chlorides and cross-linkedpolyethylenes can be characterized as being distensible in that theygrow in volume or stretch with increasing pressure until they break.These materials are generally elastic and/or stretchable. When suchextensible materials are used as medical balloons, the working diameteror designated dilation diameter of the balloon can be exceeded, basedupon the stretchability of the material.

Substantially non-distensible balloons have at times been considered tobe advantageous because they will not inflate significantly beyond theirrespective designated dilation diameters, thereby minimizing possibleover-inflation errors. The theory is that one need only select a balloonsuch as an angiographic balloon that has an opened and inflated diameterwhich substantially corresponds to the dilation size desired at theobstruction site. However, physiological vessels such as arteries aregenerally tapered and do not always coincide with readily availablecatheter balloon dimensions, and at times it may be preferable to beable to increase the diameter of the balloon beyond what had beeninitially anticipated. With a substantially non-distensible balloon,such further extension is severely limited. On the other hand, certainnon-distensible materials out of which medical balloons are madegenerally possess relatively high tensile strength values, which istypically a desirable attribute, especially for dilating tough lesions.

More readily distensible materials such as polyvinyl chloride typicallyexhibit a lower tensile strength and a larger elongation than asubstantially non-distensible material such as polyethyleneterephthalate. This relatively low tensile strength increases the riskof possible balloon failure. Due to their larger ultimate elongation,most readily distensible materials can provide a wide range of effectiveinflation or dilation diameters for each particular balloon size becausethe inflated working profile of the balloon, once achieved, can befurther expanded in order to effect additional dilation. But this veryproperty of having an expanded dilation range is not without its dangersbecause of the increased risk of overinflation that can damage the bloodvessel being treated, and the overinflation risk may have to becompensated for by using balloon wall thicknesses greater than mightotherwise be desired.

Although a material such as polyethylene terephthalate is advantageousfrom the point of view of its especially high tensile strength and itstightly controllable inflation characteristics, it has undesirableproperties in addition to its general non-distensibility. In somesituations, biaxial orientation of polyethylene terephthalate willimpart excessive crystallinity to an angioplasty balloon, or the Young'smodulus will be simply too high. Under these circumstances, the balloonitself or the thicker leg sections thereof will not readily fold overand down in order to provide the type of low profile that is desirablefor insertion through a guiding catheter and/or through thecardiovascular system and especially through the narrowed artery whichis to be dilated. The resistance to folding, or "winging," is anespecially difficult problem when it comes to larger balloons, such asthose intended for valvuloplasty applications.

Also, it has been observed that thin-walled materials such aspolyethylene terephthalate have a tendency to form pin holes or exhibitother signs of weakening, especially when flexed. Such a tendency canrequire extreme care in handling so as to avoid inadvertent damage thatcould substantially weaken a polyethylene terephthalate medical balloon.Although it is known that a lower profile and more flexible balloon andballoon legs can be made by thinning the wall, thus thinned polyethyleneterephthalate balloons become extremely fragile and may not surviveinsertion through the cardiovascular system with desired integrity andwithout pin-holding and/or rupture.

Materials such as polyethylene terephthalate do not readily acceptcoating with drugs or lubricants, which can be desirable in manyapplications. Polyethylene terephthalate materials are also difficult tofuse, whether by adherence by heating or with known biocompatibleadhesives.

By the present invention, these undesirable and/or disadvantageousproperties of substantially non-distensible medical balloons such asthose made from polyethylene terephthalate materials are significantlyeliminated. At the same time, many of the advantages of these types ofmaterials are provided or approximated. Furthermore, the presentinvention realizes many of the advantages of the more elastomericmaterials such as polyvinyl chloride, but without the disadvantages ofrelatively low tensile strength, and the possibility of excessiveexpandability that can lead to overinflation of a blood vessel or thelike.

In summary, the present invention achieves these objectives and providesadvantageous properties along these lines by forming and providing acatheter and medical catheter balloon constructed of a material that isof limited and generally controlled distensibility whereby expansionbeyond the working or fully expanded but non-distended dilation profileof the balloon is possible, but only to a desired extent which can,within limits, be tailored to the particular needs of the balloondevice. The invention includes utilizing a tailorable material such as anylon material or a polyamide material that is formed into a balloon byappropriate axial elongation, radial expansion and heat treatmentprocedures.

It is a general object of the present invention to provide an improvedmedical balloon and medical device incorporating a balloon.

Another object of the present invention is to provide an improvedmedical balloon that exhibits a controlled distensibility at highpressure.

Another object of the present invention is to provide an improvedballoon and/or catheter and method of making same which provides a rangeof dilation capabilities while minimizing risks that could be associatedwith such flexibility.

Another object of the present invention is to provide an improvedangioplasty catheter balloon molding method and apparatus that areespecially suitable for forming tubing of tailorable materials such asnylon or polyamide into catheter balloons.

Another object of the present invention is to provide an improved meansand method for radially expanding relatively amorphorous tubing intomedical balloons.

Another object of the present invention is to provide an improvedballoon catheter that is readily coated with materials such aslubricating agents and the like which are advantageous foradministration in association with balloon catheters.

Another object of this invention is to provide an improved balloon thatexhibits a combination of good strength and desirable flexibility.

Another object of this invention is to provide a medical balloon thatexhibits an ability to be readily fused to other components.

These and other objects, features and advantages of this invention willbe clearly understood through a consideration of the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of this description, reference will be made to theattached drawings, wherein:

FIG. 1 is an elevational illustration, partially in cross-section, of aballoon catheter having a structure typical of that suitable forangioplasty;

FIGS. 2a, 2b, 2c, 2d and 2e are elevational illustrations of tubing asit is progressively processed to transform same into a medical balloonaccording to this invention;

FIG. 3 is a cross-sectional view illustrating a preferred apparatus forcarrying out the processing steps illustrated in FIGS. 2a, 2b, 2c, 2dand 2e;

FIG. 4 is a plot of balloon size versus balloon inflation pressure forballoons made of a variety of materials, including nylon balloonstailored to provide different radial expansion properties;

FIGS. 5 and 6 are plots illustrating the function of heat setting ontailorability; and

FIG. 7 is a plot illustrating the function of hoop expansion ratio ontailorability.

DESCRIPTION OF THE PARTICULAR EMBODIMENTS

An illustrative catheter is generally designated in FIG. 1 by referencenumeral 21. Catheter 21 includes a catheter tube 22 having a proximalend 23 and a distal end 24. A guiding catheter 25 is also illustrated. Amedical balloon 26 is shown secured to the distal portion of thecatheter tube 22 in a location overlying one or more apertures 27through the catheter tube. Extending through the lumen of the cathetertube 22 is an inner elongated body 28, such having a proximal end 29 anda distal tip end 31. The inner body 28 may be solid or have an internallumen, depending upon the function that the inner body is to perform,whether it be simply a guiding function or whether it is intended toalso provide the capability to insert materials into the bloodstream ormeasure parameters of the bloodstream, or the like.

Except for the balloon 26, all of these various components performfunctions that are generally appreciated in the art. Typically with theaid of the guiding catheter 25, the inner body 28 and the catheter tube22 are inserted into the cardiovascular system until the balloon islocated at an occlusion site. At this stage, the balloon 26 is typicallyfolded and collapsed, and it has an external diameter less than theinflated diameter illustrated in FIG. 1, to the extent that the balloon26 is generally wrapped around the catheter tube 22. Once the balloon 26is maneuvered to the location of the occlusion, a pressurized fluid isinserted at the proximal end 23 of the catheter tube 22 for passagethrough aperture 27 and for inflation of the balloon 26. This unfoldsthe balloon until it presents a relatively smooth outer surface orworking profile for imparting forces that are radially outwardlydirected at the desired site within the body in order to achieve thedesired result of lesion dilation, occlusion reduction or similartreatment. In accordance with an important aspect of the invention, thepassage of greater pressure against the inside surface of the balloon 26permits the outer surface of the balloon 26 to transmit additional forceto the lesion or the like, which may include actual further radiallyoutwardly directed movement of the balloon 26 beyond its workingprofile, as illustrated in phantom in FIG. 1.

FIGS. 2a, 2b, 2c, 2d and 2e illustrate the transformation of a length oftubing 32 that is a material such as extruded nylon or polyamide into aballoon 33 or a modified balloon 34. Either balloon 33 or 34 possessesthe ability to be first inflated to its unextended or working profileand then therebeyond to a limited and/or controlled extent by theapplication of greater pressure. Each balloon 33 and 34 includes aballoon portion 35, 36, respectively, and leg portions 37, 38,respectively. The leg portions 37, 38 are the portions of the balloonthat are secured to the tube 22 of the catheter 21.

In an exemplary application, tubing length 32 is extruded so that itexhibits a diameter that is roughly one-quarter of the diameter intendedfor the balloon. The extruded tubing length 32 also has a nominal wallthickness that is on the order of six to twelve or so times the desiredwall thickness of the balloon 33, 34.

FIG. 2b illustrates tubing length 32a, which is tubing length 32, afterit has been elongated to approximately three times its original length.This elongation or drawing procedure is carried out at approximatelyroom temperature, and same proceeds typically until it has beenstretched to the point that it exhibits a noticeable resistance tofurther stretching. Typically, the pull force is greater than the yieldpoint of the particular tubing length 32, but less than the ultimatetensile strength, lengthwise, of the selected material. Generallyspeaking, this axial elongation procedure is carried out until the wallthickness of the tubing length 32a is roughly one-half of the wallthickness of the tubing length 32 and/or until the diameter of thetubing length 32a is roughly one-half to forty percent of the outerdiameter of the tubing length 32. The actual stretched length cantypically be about two times to about four times or more of the originaltubing length 32. Actual stretching of the tubing 32 can be performed bysimply axially stretching length 32 or by pulling or drawing length 32through a sizing die.

FIG. 2c illustrates a step that is carried out after the longitudinalelongation procedure of FIG. 2b has been completed. This is a step bywhich a portion of the tubing length 32a expands primarily radially andthereby delineates the balloon portion 35 from the leg portions 37. Thisstep is carried out by pressure exerted on the inside wall of the tubinglength 32a by a pressurized fluid. Typically, the balloon portion 35will have an outer diameter that is on the order of roughly six timesthe outer diameter of the tubing length 32a. The pressurized fluid mayinclude gasses such as compound air, nitrogen or argon. Liquids such aswater or alcohol could also be used if they do not pose a problem ofleaving residual fluid in the balloon. Generally speaking, the largerthe balloon, the faster will be the inflation fluid flow rate. Examplesof cardiac balloons generally ranked in typical order of increasing sizeare those designed for use in coronary arteries, those designed for usein peripheral arteries, and valvuloplasty balloons for use in cardiacvalves. Other balloons include uroplasty balloons for dilating theprostatic urethra.

The procedure illustrated in FIG. 2c can be facilitated by controllingthe location at which the radial expansion will occur. An advantageousmanner of effecting this result is to carry out a local application ofheat to the balloon portion 35 during radial expansion while avoidingsuch heat application at the leg portions 37. Elevating the temperaturein such a localized area will lower the yield point of the nylon,polyamide or the like at that location and thereby facilitate the ofthis selected area. As an example, a temperature for conducting thisstep typically can be between about 35° C. and perhaps as high as 90°C., the optimum temperature depending somewhat upon the particularmaterial that is utilized.

Means are provided for controlling the expansion of the tubing length32a and its formation into the balloon portion 35 and the legs 37. Thiscan be accomplished by controlling and monitoring conditions and/orexpansion positions within a molding apparatus. An exemplary means inthis regard is included in the molding apparatus described herein.Otherwise, this function can be accomplished by closely controlling therate of fluid passage and by monitoring the pressure thereof. Forexample, for a valvuloplasty balloon, the following approach can betaken. One end of the longitudinally elongated tube having a diameter ofapproximately 0.140 inch and a wall thickness of approximately 0.006inch is sealed, and a liquid such as water is pumped into the tube at arate of approximately 2 ml per minute. For tubing of this size and at aflow rate of this magnitude, balloon inflation begins at about 300 psigauge and drops to around 150 psi gauge. Expansion continues in thismanner until the wall of a mold bearing the desired shape of the balloonis encountered, which is observable by a significant pressure increase.Pumping continues until a pressure of about 180 to 200 psi is reached.This pressure condition can be held or maintained, if desired.

A satisfactory balloon can be prepared by proceeding with the methodthrough and including the step illustrated in FIG. 2c, followed bydimensional stabilization, heat setting or thermoforming, the balloon tonear its radially expanded profile and size by maintaining its elevatedtemperature until the selected material is thermally set. Tailorabilitythat is achieved according to this invention is a function of theparticular heat setting conditions. The setting temperature can varywith the type of material used, the wall thickness, and the treatmenttime. A typical setting temperature is between about 100° C. and about160° C. for a nylon material, and a typical time is from about 1 minuteto about 4 minutes.

Such balloons 33 may include expansion knurls 39 that tend to appear inthe areas generally extending between the legs 37 and the workingsurface of the balloon portion 35. These expansion knurls, which may bedescribed as nodules, ridges, ribs, beads or the like, are believed toresult from disproportionate expansion of the material such as nylon inthis area. When it is desired to minimize the existence of theseexpansion knurls 39 on the balloon, secondary longitudinal elongationwith radial shrinkage followed by secondary radial expansion can proceedwith a balloon that is not thermally set, such being illustrated in FIG.2d and FIG. 2e.

Regarding FIG. 2d, the balloon 33 of FIG. 2c, which is not thermallyset, is again longitudinally elongated by applying an axially directedforce that is typically greater than the yield point but less than theultimate tensile strength of the balloon 33. If desired, the magnitudeof this axial force can be substantially the same as the magnitude ofthe axial force applied in the procedure illustrated in FIG. 2b. Furtherexpansion is then conducted by, for example, introducing a pressurizedfluid into the balloon lumen in order to prepare the balloon 34 shown inFIG. 2e. It is often desirable to conduct this procedure within a moldcavity in order to thereby effect a careful shaping of the balloonportion 36, the leg portions 38, and the tapered connection surfacestherebetween, generally as desired. The resulting modified balloon 34typically will not include any significant expansion knurls 39, and itwill exhibit a uniform transition between the balloon portion 36 and theleg portions 38.

Whether the procedure is utilized that forms the balloon 33 or if theprocedure is continued such that the balloon 34 is formed, the thusformed balloon 33, 34 is preferably then subjected to a heat settingstep at which the still pressurized balloon 33, 34 is heated to set theexpanded dimensions thereof. Setting will typically be accomplished at atemperature of 85° C. or greater, typically up to about the meltingpoint of the balloon material, depending somewhat upon the actualmaterial and the size and thickness of the balloon. For example, atypical heat setting temperature for Nylon 12 is 1 minute at 120° C.With this heat treatment, the balloon will retain this form and most ofits expanded size, typically about 95% of more of it, upon being cooled.Preferably, the balloon remains pressurized until it is generallycooled. If this heat setting procedure is not utilized, the balloon willpromptly shrink back to approximately 40% to 50% of the diameter towhich it had been blown or biaxially oriented in the mold.

With more particular reference to this heat setting procedure, animportant component in this regard is the use of a thermoformablematerial. A material is thermoformed if it can be formed to anothershape. Nylon is a thermoforming plastic. It can be heated to atemperature below its melting point and deformed or formed to take onanother shape and/or size. When cooled, thermoforming materials retainthat new shape and/or size. This procedure is substantially repeatable.On the other hand, a thermosetting material, such as a natural latexrubber, a crosslinked polyethylene or a silicone rubber, once "set" iscrosslinked into that size and/or shape and cannot be heat deformed.Also, biaxially oriented polyethylene terephthalate tends to crystallizeand lose the thermoforming ability that is typical of polyethyleneterephthalate which is not biaxially oriented. When a nylon balloon asdiscussed herein is heated for a prolonged period of time attemperatures greater than a predetermined elevated temperature, forexample about 80° C., and then cooled, it becomes thermoformed to theballoon geometry. If the balloon were to be reheated to this temperatureor above, it could be formed into a balloon having a different geometry.

FIG. 3 illustrates a molding apparatus that is suitable for fabricatingmedical balloons as discussed herein. The apparatus transforms a parison41 into balloons for medical devices and the like. The apparatusachieves longitudinal stretching, radial expansion, heating and cooling,and it includes means for monitoring radial expansion, all of which canbe conveniently controlled by suitable means such as hard circuitry, amicroprocessor, or other computerized controlling arrangements. Thesevarious parameters that are controlled can thus be precisely set andeasily modified in order to present the optimum conditions forfabricating a particular parison into a balloon having a specifiedsizing and the properties desired. Specific parameter values arepresented herein which are typically suitable for balloon materials suchas nylons, and it will be understood that these parameter values can bemodified as needed for the specific material being shaped into theballoon.

The apparatus that is illustrated also has the ability to generallysimultaneously perform different steps on multiple balloon portions ofthe parison 41 as it passes through the apparatus. This can beespecially useful because of variations in wall thickness and otherattributes of each batch of tubing that is used as the parison 41.

The apparatus includes a series of components which are suitably mountedwith respect to each other, such as along slide rails 42. From time totime, movement of some of the components along the rails or the like iscarried out by suitable means, such as piston assemblies (not shown) inaccordance with generally known principles and structures.

A pressurized fluid supply 43 is provided to direct pressurized fluidinto an end portion of the parison 41, typically in conjunction with oneor more regulators 44 in accordance with well-known principles. One ormore valves 45 also assist in controlling the flow rate and volume ofpressurized fluid within the parison 41. Gripper assemblies 46 and 47are provided for engaging different locations of the parison 41. Anintermediate gripper assembly 48 is preferably also provided. One ormore chiller chambers 51, 52, 53, 54 are also preferably provided. Theseare particularly useful as means for thermally isolating assemblies ofthe apparatus from each other. Also included are a free-blow radialexpansion chamber or mold 55 and a molding chamber 56.

Preferably, means are provided by which the temperature of these variouschambers is controlled and/or varied, for example between about 0° C.and about 150° C. or more. The illustrated assembly in this regardincludes a jacket for confining a thermal fluid that is pumped thereintoand out thereof by suitable fluid inlets 57 and fluid outlets 58.O-rings 59 or the like are provided in order to contain the thermalfluid within each fluid jacket 61, 62, 63, 64, 65, 66.

In using the illustrated apparatus, the parison 41 is initially fed orthreaded through the entire apparatus, typically between gripperassembly 46 and gripper assembly 47. Gripping pads 68 of the gripperassembly 47 move inwardly and engage the downstream end portion of theparison 41 to the extent that it pinches off this portion of the parison41, preferably also heat sealing the parison at this time. Generallysimultaneously, gripping pads 67 of the gripper assembly 46 engage theparison 41 at the illustrated upstream location of the apparatus to suchan extent that the gripping pads 67 will prevent movement of the parison41 with respect to the gripping pads 67, but still permit the flow ofpressurized fluid thereacross when desired. Gripper assembly 47 thenmoves in a downstream direction (to the right as illustrated in FIG. 3)until the length of the parison that is secured between gripperassemblies 46 and 47 is stretched to in excess of twice its length up toas great as about four times its length or more, a typical stretchingbeing approximately three times this unstretched length.

Next, the free-blow radial expansion chamber 55 is heated by passingheated thermal fluid into the fluid jacket 62. The chiller chambers 51,52, or other suitable means, provide a thermal variant such that theheat from the fluid jacket 62 is imparted only to that length of theparison that is substantially within the free-blow chamber 55. Thetemperature of this particular portion of the parison 41 will be heatedto a temperature of roughly between about 70° C. and 120° C. or more,depending upon the particular parison 41 and the balloon propertiesdesired. At this time, pressurized fluid within the parison 41 thatoriginates from the supply 43 passes through the parison length at thegripper assembly 46 and into the parison length at the free-blow chamber55. If desired, the gripper assembly 48 can be utilized in order toconfine this particular pressure application to the section of theparison 41 that is upstream thereof.

A primary objective of the free-blow radial expansion chamber 55 is toradially expand that portion of the parison 41 into a balloon 33 asgenerally illustrated in FIG. 2c. It is often desired to preciselycontrol the amount of biaxial orientation, and means in this regard areprovided. The means illustrated in FIG. 3 includes a slidable insert orring 69. When this insert 69 is engaged by the expanding balloon 33, itmoves against an air or metallic spring (not shown) in a direction tothe left as illustrated in FIG. 3 until it trips a suitable switch orthe like (not shown), which signals that the desired degree of radialexpansion has been achieved. At this time, steps are taken to interruptthe radial expansion. This typically includes cooling by exchanging theheated thermal fluid within the fluid jacket 62 for thermal fluid havinga colder temperature, typically on the order of about 10° C. or somewhatabove or below depending upon the particular parison 41 and theparticular properties desired.

The pressure imparted to balloon portion 33 is then depleted by, forexample, permitting exhaustion thereof through the valve 45 after theballoon has been cooled. If utilized, the gripper assembly 48 isreleased, and the balloon portion 43 is moved from the free-blow chamber55 into the molding chamber 56 by movement of the downstream end portionof the parison by the gripper assembly 47. This typically simultaneouslyaccomplishes the longitudinal elongation stage depicted in FIG. 2d. Oncethis positioning takes place, the pressurized fluid is pumped into thatportion of the parison that is within the molding chamber 56, andthermal fluid is passed into the fluid jacket 65 in order to heat thisportion of the parison to an elevated temperature, again between about70° C. and up to just below the melting point, for example 150° C. ormore, depending upon the particular parison and the properties desiredof the balloon.

Generally speaking, it is usually advantageous that the temperature inthe molding chamber 56 be higher than that applied in the free-blowchamber 55, while at the same time imparting a pressure to the insidewalls of the parison within the molding chamber 56 that is equal to orlower than the pressure applied in the free-blow chamber 55. Forexample, when a nylon is the material, the temperature in the free-blowchamber 55 can be slightly above ambient, preferably in a range ofbetween about 30° C. and 60° C., while the temperature in the moldingchamber 56 can be at the high end of this range or even well above, asneeded. Exemplary pressures would include on the order of tenatmospheres in the molding chamber 56 and twice that pressure or greaterin the free-blow chamber 55. The exact pressure is determined by thematerial and by the wall thickness and hoop stress of the balloon to bemolded.

With heat thus imparted to the modified balloon 34 within the moldingchamber 56, the balloon 34 is thereby thermoformed, with heat setting inthis regard involving raising the temperature of the thermoplastic whileit is under inflated stress. Thereafter, the heated fluid within thefluid jacket 65 is exchanged for cooling fluid in order to substantiallymaintain the size and shape of the balloon 34 formed within the moldingchamber 56. After the pressure has been relieved, the balloon is removedfrom the apparatus. Subsequently, the thus modified parison is severedgenerally along lines A and B as illustrated in FIG. 2e in order tothereby form the balloon 26 for inclusion within a medical device suchas the catheter 21.

It will be appreciated that the parison will include balloons in variousstages of their formation as the parison passes through the apparatus.Preferably, this is accomplished in a somewhat reverse manner, asfollows. After initial stretching and formation of a balloon 33 withinthe free-blow chamber 55, the apparatus is utilized so that the portionof the parison within the molding chamber 56 is radially expanded beforethat within the free-blow chamber 55, which generally occurs as follows.Molding chamber 56 is heated as described herein and pressurized to formthe balloon 34. Thereafter, the gripper assembly 48 closes off theparison between the chambers 55 and 56. The free-blow chamber 55 is thenheated, and the radial expansion is carried out as described herein inorder to form the balloon 33. Balloon 33 is then moved into the moldingchamber 56, and the process is essentially repeated.

It should be appreciated that the balloon can be made entirely in themold section 56 without free blowing in compartment 55. However,balloons made in this manner may demonstrate knurling.

Regarding the fluids suitable for use in the apparatus, it is typicallyadvantageous to have the pressure source 43 provide a fluid that doesnot require excessive treatment to remove same from the internalsurfaces of the finished medical device balloon 26. To be taken intoconsideration in this regard are moisture content of fluids and ease andsafety of disposal of the fluid. A particularly suitable fluid ispressurized nitrogen gas. With respect to the fluid for use within thevarious fluid jackets 61 through 66, it is typically best to utilize afluid that is in its liquid state throughout whatever processingtemperatures might by needed for the apparatus. Preferably the fluid isone that will likewise maintain a generally consistent viscositythroughout the processing range, for example, avoiding the onset ofsolidification or crystal formation or the like that would modify theproperties of the thermal fluid at lower processing temperatures.

With respect to the materials out of which the balloon and parison aremade, they are typically nylons or polyamides. Preferably, thesematerials have an amorphous nature while still possessing enoughcrystallinity to be formed under the conditions provided according tothis invention. The materials should also have substantial tensilestrength, be resistant to pin-holing even after folding and unfolding,and be generally scratch resistant. The material should have anintrinsic viscosity that enables blowing at elevated temperatures. Atypical intrinsic viscosity range that is preferred for the nylon orpolyamide materials according to this invention is between about 0.8 andabout 1.5, with about 1.3 being especially preferred. It is also desiredthat the material have a reasonably good moisture resistance. Materialsof the Nylon 12 type have been found to possess these advantageousproperties. Other exemplary nylons include Nylon 11, Nylon 9, Nylon 69and Nylon 66.

With more particular reference to the intrinsic viscosity of nylon orpolyamide materials, it is believed that such materials are able togenerally maintain their intrinsic viscosities during extrusion andthrough to final balloon fabrication. It is further understood thatother materials such as polyethylene terephthalate which have been usedto fabricate balloons for medical devices typically do not exhibit thismaintenance of intrinsic viscosity, but the intrinsic viscosity of thepellet material drops substantially during extrusion, with the resultthat a biaxially oriented polyethylene terephthalate balloon isunderstood to have an intrinsic viscosity which is lower than that ofthe polyethylene terephthalate pellets from which it originated. It isfurther believed that a higher balloon intrinsic viscosity reduces thelikelihood that pin-holing will develop in the finally fabricatedballoon.

With further reference to the nylon or polyamide materials which may beused according to this invention, they are bondable to the catheter 21by epoxy adhesives, urethane adhesives, cyanoacrylates, and otheradhesives suitable for bonding nylon or the like, as well as by hot meltbonding, ultrasonic welding, heat fusion and the like. Furthermore,these balloons may be attached to the catheter by mechanical means suchas swage locks, crimp fittings, threads and the like. Nylon or polyamidematerials can also be provided in the form in which they are reinforcedwith linear materials such as Kevlar and ultra high tensile strengthpolyethylenes. The polymer materials used can also be coated withpharmaceutical materials such as heparin and the like, non-thrombogeniclubricants such as polyvinyl pyrrolidone and the like. Additionally, thepolymer materials can be filled with radiopaque media such as bariumsulfate, bismuth subcarbonate, iodine containing molecules, tungsten, orother fillers such as plasticizers, extrusion lubricants, pigments,antioxidants and the like.

Nylons or polyamides as used according to the present invention areflexible enough to tolerate a greater wall thickness, even in the legareas, while still providing a structure that is foldable onto itselffor insertion into a body cavity or guiding catheter. These nylons orpolyamides, when formed into balloons as discussed herein, have acalculated tensile strength of between about 15,000 and about 35,000 psiand above, preferably between about 20,000 and about 32,000 psi.

The materials according to the present invention form medical deviceballoons that exhibit the ability to be expanded first to anon-stretched or non-distended condition, or working size, upon theapplication of a given pressure. They also have the ability to beinflated further so as to be stretched therebeyond in a controlled andlimited manner. The degree of such stretching or expansion can, withinlimits, be tailored as needed. In other words, the balloons according tothe present invention allow for some growth at a pressure higher thanthat needed to merely fill the balloon, but this additional growth orexpansion is not so substantial that there is an overinflation danger.

Materials according to this invention should be able to be tailoredduring balloon formation to possess the ability to be stretched agenerally predetermined percentage beyond its non-distended or workingdiameter, the amount of this percentage depending upon conditions underwhich the parison was processed into the balloon. A suitable materialaccording to the present invention can be tailored to cover valueswithin a span of at least 10 percentage points of radial expansion.Certain materials including some nylons can exhibit a tailorabilityrange of 25 or 30 percentage points or more and can, for example,exhibit a radial expansion of 10 percent or below and as high as about30 percent or above.

In order to illustrate this situation, tests were run on angioplastyballoons of different materials, namely polyvinyl chloride, cross-linkedpolyethylene, polyethylene terephthalate and nylon. These data arereported graphically in FIG. 4. Wall thicknesses varied depending uponthe properties of the material in order to provide a viable balloon. Thepolyethylene terephthalate balloon (B) had a wall thickness of 0.010 mm.The polyvinyl chloride balloon (C) had a wall thickness of 0.064 mm, andthe polyethylene balloon (D) had a wall thickness of 0.058 mm. (Data forballoons C and D are as reported in "Effects of Inflation Pressure onCoronary Angioplasty Balloons," The American Journal of Cardiology, Jan.1, 1986, these tests being run under ambient conditions, while theremaining balloons in FIG. 4 were tested at 37° C.) One of the nylonballoons (A) had a wall thickness of 0.013 mm and a hoop expansion ratioof 5.2. Another of the nylon balloons (E) had a wall thickness of 0.008mm and a hoop expansion ratio of 4.3. Tailorability of balloonsaccording to this invention is a function of hoop expansion ratio, whichis defined as mean balloon diameter divided by mean as-extruded tubingor parison diameter.

FIG. 4 plots the relative pressure of the fluid imparted to the variousangiographic catheters against the percentage increase in size (such asdiameter or radius) of the balloon beyond its working size, designatedas "0." It will be noted that the more compliant materials, such aspolyvinyl chloride, gradually grow with increased pressure and permitsome limited additional pressure increase after the working size hasbeen achieved and before further expansion ceases prior to reaching theburst limit of the balloon. It will be appreciated that, with materialssuch as these, a smaller increase (when compared with materials of plotsA and B) in relative pressure significantly increases the balloon size.Materials such as polyethylene terephthalate (PET) inflate to theirworking size, but not therebeyond to an extent as great as others of theplotted curves.

The nylon or polyamide material illustrated by curve E exhibits arelatively long and flat curve in FIG. 4, which indicates that it willcontinue to expand moderately upon the application of relatively smalladditional increments of inflation pressure. Once the unstretched ornon-distended profile or working size is reached, this tailored nylonballoon possesses adequate stretchability or compliance to the extentthat it can ramp up to another inflated profile. While the polyethyleneballoon of curve D initially distends to about the same extent as doesthe nylon curve E balloon, this nylon balloon can tolerate greaterinflation pressures without bursting than can the curve D polyethyleneballoon or the curve C polyvinyl chloride balloon. This observation isespecially significant when it is appreciated that the curve E nylonballoon had a substantially thinner wall diameter. Based upon data suchas that illustrated in FIG. 4, it is possible to predict the compliantexpansion associated with added inflation pressure. A balloon inaccordance with the present invention can exhibit this growth propertyso that it will radially expand from a low of about 5 percent beyond theworking size to a high of about 35 percent or more of the working size.

As previously observed herein, balloon expansion tailorability is afunction of heat setting conditions and of hoop expansion ratio forballoon materials according to the present invention. For suchmaterials, increasing the heat set temperature increases the size of thefinished balloon at a given inflation pressure and decreases theshrinkage which typically occurs after sterilization such as a standardethylene oxide treatment. Also, the amount of hoop expansion imparted tothe parison during processing has a significant effect on the amount ofdistention which the finished balloon will exhibit.

FIG. 5 and 6 illustrate the effect of varying heat setting conditions onthe distensibility of the finished balloon. This heat setting isachieved when the balloon is subjected to a temperature greater than theglass transition temperature of the balloon material and is allowed tocool to below the glass transition temperature. For a material such as anylon or a polyamide, temperatures between about 90° C. and about 180°C. have a pronounced heat setting capability. The glass transitiontemperatures may be less than this heat setting temperature and mayinclude temperatures in the range of 20° C. to about 40° C., and theballoon will exhibit good flexibility at body temperature which can, forexample, facilitate balloon insertion and maneuverability during use.

FIG. 5 gives four distention plots, tested at 37° C. for nylon balloonsthat are substantially identical except they were subjected to differentheat set temperatures. In each case, the balloon was subjected to a 2minute heat/cool cycle, with cooling being to room temperature. Eachballoon was subsequently subjected to ethylene oxide sterilization.Curve F was subjected to a heat set temperature of 120° C., curve G wasat 130° C., curve H was at 140° C., and curve I was at 150° C. It willbe noted that the curves illustrate different distention properties andthe type of tailoring thus achieved. FIG. 6 provides a similarillustration and further shows an effect of the ethylene oxidesterilization. While curves J and K both are for balloons which wereheat set at 100° C., the curve J balloon was sterilized while the curveK balloon was not. Likewise, the balloons of curves L and M both wereheat set at 140° C., curve L illustrating a sterilized situation, andcurve M a non-sterilized situation.

The relationship between balloon tailorability and hoop expansion ratiois illustrated in FIG. 7. Three non-sterilized nylon balloons havingdifferent hoop expansion ratios were tested at 37° C., the expansionbeing to burst. A relatively high hoop expansion ratio of 4.9 (curve N)gave a balloon distention (safely short of burst) of about 7 percent. A3.7 hoop expansion ratio (curve O) gave a balloon distention of about 23percent, while a 3.3 hoop expansion ratio (curve P) gave a balloondistention of about 36 percent.

Furthermore, nylon or polyamide balloons according to the presentinvention provide controlled expansion properties and tolerate givenpressures without bursting without requiring wall thicknesses as greatas those needed for other materials such as polyvinyl chloride orpolyethylene. This thin wall thickness of these nylon balloons permitsthe balloon to enter smaller vessels, lesions and/or catheters. Suitableballoons according to this invention can have shell or wall thicknessesas low as about 0.002 inch and as high as 0.002 inch. An exemplarypreferred shell thickness range is between about 0.004 and about 0.001inch, or between about 0.005 and about 0.008 inch.

A material such as nylon is softer and more flexible at bodytemperature, the temperature at which it is used, than other balloonmaterials such as polyethylene terephthalate which have a glasstransition temperature far in excess of body temperature. For at leastsome nylons, the glass transition temperature approximates bodytemperature, and nylon balloons generally exhibit amorphous propertiesthat enhance flexibility and softness even after proceeding with thetype of processing described herein, including thermoforming and thelike. Nylon balloons thus can be at a glass transition state while theyare within the body, which is not the case for numerous other materialsthat can be used as medical balloons.

Balloons made of nylons exhibit a property such that, when they burst,the diameter of the balloon will decrease, thereby facilitating removalfrom the body. It is believed this decrease may be caused by a relaxingof the material which noticeably reduces the size of the balloon. Othermaterials such as polyethylene terephthalate are not known to exhibitthis advantageous property. A disadvantageous property that can becharacteristic of polyethylene terephthalate medical balloons is thedevelopment of extensive material wrinkling upon sterilization, whichwrinkles persist, even at body temperature, until very high inflationpressures are applied, on the order of 200 psi. When a nylon orpolyamide balloon is sterilized, few if any material wrinkles develop,and these substantially disappear at relatively low inflation pressures,on the order of 8 psi.

Nylon materials have been observed to exhibit desirable stability duringprocessing to the extent that they do not absorb excessive moisture fromthe environment if the parison is allowed to stand uncovered forreasonable time periods. Materials such as polyethylene terephthalateare degraded by environment moisture if the parison is not stored inprotective bags or the like prior to formation of the balloon.

It will be understood that the embodiments of the present inventionwhich have been described are illustrative of some of the applicationsof the principles of the present invention. Numerous modifications maybe made by those skilled in the art without departing from the truespirit and scope of the invention.

We claim:
 1. A dilatation balloon, comprising:a length of thermoformablepolymeric material tubing that had been radially expanded to apredetermined balloon diameter, said thermoformable polymeric materialhaving a glass transition temperature of between about 20° C. and about40° C.; said dilatation balloon has a collapsed profile which has adiameter less than said predetermined balloon diameter, said dilatationballoon further having a non-distended, inflated working diameter atwhich the dilatation balloon is inflated to its said predeterminedballoon diameter, and said dilatation balloon also exhibits a maximumstretched inflation diameter to which the dilatation balloon isexpandable without bursting under needed dilatation conditions, saidmaximum stretched inflation diameter being in excess of saidpredetermined diameter; and said dilatation balloon has a calculatedtensile strength of at least about 15,000 psi.
 2. The dilatation balloonaccording to claim 1, wherein the balloon is at a glass transition stateof the thermoformable polymeric material and exhibits enhancedflexibility while within a human body.
 3. The dilatation balloonaccording to claim 1, wherein said maximum stretched inflation diameterexceeds said predetermined balloon diameter by between above 5 percentand about 30 percent.
 4. The dilatation balloon according to claim 1,wherein said maximum stretched inflation diameter exceeds saidpredetermined balloon diameter by between above 5 percent and about 40percent.
 5. The dilatation balloon according to claim 1, wherein saidtubing had been subjected to at least two radial expansion procedures,and wherein said dilatation balloon is substantially free of expansionknurls.
 6. The dilatation balloon according to claim 1, wherein saidtubing material of the dilatation balloon has tailorability propertieswhereby said dilatation balloon has a maximum stretched inflationdiameter which had been selected during balloon formation.
 7. Thedilatation balloon according to claim 6, wherein said maximum stretchedinflation diameter of the balloon is at least about 10 percentage pointsof radial expansion.
 8. The dilatation balloon according to claim 1,wherein said maximum stretched inflation diameter of the balloon isgreater than about 5 percent in excess of the predetermined balloondiameter.
 9. The dilatation balloon according to claim 1, wherein saidballoon has an intrinsic viscosity of between about 0.8 and about 1.5.10. The dilatation balloon according to claim 1, wherein saidthermoformable polymeric material is a nylon material or a polyamidematerial.
 11. The dilatation balloon according to claim 1, wherein saidthermoformable polymeric material comprises a Nylon 12 type of material.12. The dilatation balloon according to claim 1, wherein saidthermoformable polymeric material includes a nylon selected from thegroup consisting of Nylon 12, Nylon 11, Nylon 9, Nylon 69 and Nylon 66.13. The dilatation balloon according to claim 1, wherein said balloon isan angioplasty balloon.
 14. A dilatation catheter, comprising:a catheterbody; a dilatation balloon member positioned along the length of saidcatheter body; said balloon member being constructed of a thermoformablepolymeric material tubing that had been radially expanded to provide apredetermined balloon diameter, said thermoformable polymeric materialhaving a glass transition temperature between about 20° C. and about 40°C.; said balloon member has a collapsed profile at which the outerdiameter of the balloon member approximates the outer diameter of saidcatheter body, said balloon member further having a non-distendedworking diameter to which the balloon is inflated to its saidpredetermined balloon diameter without significant stretching thereof,and said balloon member also exhibits an expanded maximum inflateddiameter to which the balloon member is stretched without burstingduring dilatation, said maximum inflated diameter being in excess ofsaid predetermined balloon diameter; and said dilatation balloon memberhas a calculated tensile strength of at least about 15,000 psi.
 15. Thedilatation catheter according to claim 14, wherein the balloon is at aglass transition state of the thermoformable polymeric material andexhibits enhanced flexibility while within a human body.
 16. Thedilatation catheter according to claim 14, wherein said maximum inflateddiameter exceeds said predetermined balloon diameter by between above 5percent and about 30 percent.
 17. The dilatation catheter according toclaim 14, wherein said maximum inflated diameter exceeds saidpredetermined balloon diameter by between in excess of 5 percent andabout 40 percent.
 18. The dilatation catheter according to claim 14,wherein said tubing material of the dilatation balloon has tailorabilityproperties whereby said dilatation balloon has a maximum stretchedinflation diameter which had been selected during formation of thedilatation balloon.
 19. The dilatation catheter according to claim 18,wherein said maximum stretched inflation diameter of the balloon is atleast about 10 percentage points of radial expansion.
 20. The dilatationcatheter according to claim 14, wherein said maximum stretched inflationdiameter of the balloon is greater than about 5 percent in excess of thepredetermined balloon diameter.
 21. The dilatation catheter according toclaim 14, wherein said balloon member material has an intrinsicviscosity of between about 0.8 and about 1.5.
 22. The dilatationcatheter according to claim 14, wherein said thermoformable polymericmaterial is a nylon material or a polyamide material.
 23. The dilatationcatheter according to claim 14, wherein said thermoformable polymericmaterial comprises a Nylon 12 type of material.
 24. The dilatationcatheter according to claim 14, wherein said thermoformable polymericmaterial includes a nylon selected from the group consisting of Nylon12, Nylon 11, Nylon 9, Nylon 69 and Nylon
 66. 25. The dilatationcatheter according to claim 14, wherein said balloon is an angioplastyballoon.